A total hip prosthesis generally includes two parts: a first part integrally connected to the femur, known as the femoral part comprising a rod introduced in the femur, equipped with an essentially spherical head, and an acetabulum provided to receive the femoral head. The acetabulum, also known as a cup when it is hemispherical, is positioned in the corresponding housing (the anatomical acetabulum) of the hip bone.
Implantation of a prosthesis by a surgeon is a relatively complex operation, since the femoral part and the acetabulum even more so, must be placed in an optimized manner, particularly to prevent the prosthesis from dislocating during high amplitude movements.
According to conventional methods, the pelvis is palpated to locate the three points of the anterior pelvic plane (APP). This anterior pelvic plane (also known as the Lewinneck plane) is a frame of reference conventionally utilized in hip surgery. It is defined by the two iliac spines and by the pubic symphysis. This plane enables the prosthetic acetabulum to be adequately oriented in terms of inclination and anteversion.
The surgeon then inserts the acetabulum or cup at the end of a tool known as an impactor. He manipulates this cup so as to place it such that it presents an inclination of 45° and an anteversion of 15° with relation to the anterior pelvic plane.
However, these two angle values are average values, utilized by default, and do not correspond to all particular positions likely to be encountered.
An improvement to this approach was proposed in the document of patent U.S. Pat. No. 6,205,411, that proposes a computerized simulation of an optimized prosthesis implantation from a tomography of the bone envelope of the pelvis and femur, done before the operation.
During the operation, the surgeon is guided in placing the acetabulum, by using an inner body placed on the pelvis and the femur to ensure location in space, according to the simulation result.
This result is effective but presents the disadvantage of significant complexity (tomography, computerized simulation, etc.) that limits its use, particularly for reasons of cost.
Another approach was proposed in the document of patent FR-2 865 928. According to this technique, a “mega-head” is used, placed in the cotyloid cavity hollowed in the pelvis. A processing device enables a mobility cone and extreme positions to be simultaneously displayed, according the center of the cup and the geometry of the femoral prosthesis.
The surgeon may then manipulate the cup by using an impactor to bring the extreme positions into the mobility cone.
This technique is more simple than that described in document U.S. Pat. No. 6,205,411, and does not necessitate prior measurements. However, this technique may turn out to be insufficient in practice, since the measurements are performed during surgery, and the individual is put to sleep in a particular position (recumbent position).